This invention relates to photorefractive systems and methods and more particularly to systems, devices, and methods using photorefractive media to form readable, permanent, high resolution, and high information content images, and to provide specialized optical devices.
The photorefractive effect has been known for many years, as pointed out in an article by David M. Pepper et al., entitled "The Photorefractive Effect," Scientific American, October 1990, pp. 62-74. The term "photorefractive" is applied to electro-optically active materials, such as what are generally referred to as the ferroelectrics, which exhibit a large response to incident electromagnetic wave energy. The useful materials are crystals of substantial thickness in contrast to films and layers, and are particularly suitable for true volume holographic applications. The role and history of the ferroelectrics in prior investigations until publication date was described in the book Volume Holography and Volume Gratings by L. Solymar and D. J. Cooke (New York, Academic Press Inc., 1981). This publication discusses not only theoretical considerations but also practical problems that have been encountered with volume holography in general and photorefractive holographic systems in particular.
Photorefractive materials have the property of developing light-induced changes in their index of refraction. This property can be used to store information in the form of holograms by establishing optical interference between two coherent light beams within the material. The spatial index of refraction variations are generated through the electro-optic effect as a result of an internal electric field generated from migration and trapping of photoexcited electrons. While many materials have this characteristic to some extent, the term "photorefractive" is applied to those which have substantially faster and more pronounced response to light wave energy. Examples of such materials include LiNbO.sub.3 (lithium niobate), LiTaO.sub.3, Li.sub.2 B.sub.2 O.sub.4 (lithium borate), KNbO.sub.3, KTa.sub.1-x Nb.sub.x O.sub.3 (KTN), strontium barium niobate (SBN), barium strontium potassium sodium niobate (BSKNN or KNSBN), Bi.sub.12 SiO.sub.20 (BSO), and BaTiO.sub.3.
Soon after identification of this effect, it was recognized that by using the photorefractive effect together with holographic techniques, one can generate a volumetrically distributed pattern in the material in the form of a refractive index grating. The stored hologram can then be read out with a reference beam, as in other holographic systems. However, since the holographic gratings are distributed over a volume, the higher order diffraction terms that are present in thin holograms are suppressed.
A prominent early proposal for the use of photorefractive properties is contained in U.S. Pat. No. 3,544,189 to Fang-Shang Chen et al., entitled "Holography Using a Poled Ferroelectric Electric Recording Material." Chen, et al. described how electrons migrate along the electro-optic or c-axis in a lithium niobate crystal in response to varying light intensity, where they concentrate in low light intensity regions and define a charge distribution pattern. The localized electric fields generate local changes in the index of refraction through the electro-optic effect, creating a three-dimensional hologram in the form of an index grating distributed throughout the volume of the medium. Chen, et al. described how a multiplicity of holograms can be recorded in the same thick plane of material by using different beam angles, which can be done by turning the crystal or the beams through successive, very small angles. Using these techniques, workers in the art have calculated that a 1 cm.sup.3 crystal is theoretically capable of storing 10.sup.12 to 10.sup.13 bits of information. Chen, et al. recognized that the images formed in this manner are impermanent (metastable) in the sense that they diminish with time and can be altered by incident light. They also proposed erasure of the image by heating it to at least 160.degree. C. The technique of Chen, et al. forms transmission mode gratings or holograms because the reference and object beams are both incident on the same face of the crystal.
The concentration as well as type of dopant can be changed to achieve specific desired properties in the crystal. The photorefractive band of the crystal can be shifted, the sensitivity can be changed, and the absorption can be increased or decreased (see W. Phillips, J. J. Amodei, and D. L. Staebler, "Optical and holographic storage properties of transition metal doped lithium niobate," RCA Review vol. 33, pp, 94-109 (1972)). For example, the typical photorefractive band for 0.05% Fe-doped lithium niobate is in the range of 400 to 800 nm, but absorption and photorefractive properties can be shifted with higher dopant levels or by oxidation or reduction of the dopant in the crystal.
The fact that the recorded holograms are metastable, decaying even in the dark with time, has resulted in the development of techniques for fixing and developing permanent holograms. Metastable holograms formed by the electron charge patterns can be made permanent by heating the crystal to displace ions into a compensating pattern and then redistributing the electrons substantially uniformly throughout the volume. The method used was to heat the crystal containing the metastable electronic hologram so the ions redistributed themselves to cancel the space charge variation in the crystal. Then the crystal was cooled and the electronic grating erased by intense illumination, leaving only the permanent ionic grating to generate the index variation. Lifetimes of up to 10.sup.5 years were projected by extrapolating erase times measured at elevated temperatures (J. J. Amodei and D. L. Staebler, "Holographic recording in lithium niobate," RCA Review vol. 33, pp. 71-93 (1972)).
As discussed by W. Phillips et al., supra, however, the techniques currently used result in a generally inverse relation between photorefractive sensitivity and the diffraction efficiency of the holograms after fixing. The higher the dopant concentration, the more electron donors and the higher the sensitivity and refractive index change at the output up to a level of saturation, as also shown in Yu. V. Vladimirtsev et al. in "Optical Damage in Transition Metal Doped Ferroelectrics," Ferroelectrics, vol. 22, pp. 653-654 (1978). The known fixing techniques, however, degrade the hologram so that the fixed efficiency decreases as dopant level increases. There is therefore an inherent processing limitation on the utility of photorefractive devices that effectively limits the dopant concentration and therefore the magnitude of the photorefractive effect in fixed crystals. This must be overcome if the potential of this type of optical device is to be fully realized.
The Chen, et al. patent was preceded by an earlier patent by the same authors, U.S. Pat. No. 3,383,664, directed toward digital information storage in photorefractive media. Subsequent work has included a patent to J. J. Amodei et al., U.S. Pat. No. 3,915,549, concerning the use of iron-doped lithium niobate crystals to improve the sensitivity, followed by U.S. Pat. No. 3,932,299 to W. Phillips on the reduction of the trivalent iron (Fe.sup.3+) to divalent iron (Fe.sup.2+) in the Fe-doped lithium niobate crystals so as to increase the erase sensitivity and further reduce the time required for hologram formation. J. P. Huignard et al., in U.S. Pat. No. 4,449,785, titled, "Multiple Hologram Bulk Optical Storage Device" further expanded on the angle multiplex approach of Chen, et al., using the transmission mode and lithium niobate crystals. In this patent, transparent electrodes were attached to all faces of the material and electrically short circuited during writing so as to prevent saturation of the index of refraction after several storage operations. The problem here is that the amplitude of the variations in the space charge field, which are needed for high diffraction efficiency, is not increased.
Equations describing the photorefractive effect have been derived by N. V. Kukhtarev et al., "Holographic storage in electro-optic crystals. I. Steady state," Ferroelectrics vol. 22, pp. 949-960 (1979). For a one carrier, single dopant type model, with electrons being the sole charge carrier, which is true in general for many of the previously mentioned materials, this process is described in the steady state by: ##EQU1## In the preceding equations, N.sub.D is the electron donor density, N.sub.D + is the trap density, N.sub.A is the density of non-photoactive ions, N.sub.0 is the total dopant density, J is the current density, E is the electric field, I is the light intensity, .beta. is the dark decay constant, .gamma. is the recombination rate, .mu. is the mobility, , n is the electron density, k.sub.B is Boltzmann's constant, s is the photoionization cross section, .epsilon. is the permittivity, e is the electron charge, h.omega. is the photonic energy of the light, T is the temperature (in degrees K), and t is the time.
In the prior art, the holograms were usually written in transmission mode with angular multiplexing, which is storing different holograms with different object to reference beam angles (F. H. Mok, M. C. Tackitt, and H. M. Stoll, "Storage of 500 high resolution holograms in a LiNbO.sub.3 crystal," Opt. Lett. vol. 16, pp. 605-607 (1991)), or by rotating the crystal for each new exposure while keeping the object to reference beam angle constant (W. Phillips et al., supra). Lithium niobate has often been used because of its long dark lifetime and the capability to fix the gratings. Most photorefractive work in LiNbO.sub.3 has been done with Fe-doped crystals with dopant concentrations of up to 0.015%. F. H. Mok et al. demonstrated storing 500 image-bearing holograms, each with an unfixed diffraction efficiency of approximately 0.01 in a 1 cm.sup.3 crystal (F. H. Mok et al., supra).
One would perhaps superficially assume that the earlier angle multiplexing techniques would be combined with the subsequent fixing and development methods to create high density holographic storage media capable of optical readout. However, useful three-dimensional information storage systems have not yet been implemented, as far as is known. There are many interrelated factors that explain why this is the case, and these are included in the publication Volume Holography and Volume Gratings and in the Scientific American publications, supra. The dopant level should be high for fast response time, sensitivity, and to generate substantial electrical fields, but as noted above a metastable image of relatively high diffraction efficiency cannot be made permanent at that same efficiency. In addition, there are wave mixing and coupling effects that must be taken into consideration. The hologram being generated in a photorefractive crystal, as noted by Solymar, et al., itself can affect and interact with the incident wave energy, causing the initially sinusoidal grating pattern to become highly non-sinusoidal. Waves incident at different angles relative to the preferential migration path along the c-axis introduce scattering and also generate holograms of different diffraction efficiency. Further, when holograms are written serially but coextensively within the same volume, the later holograms non-uniformly reduce the diffraction efficiencies of the previous ones under the present methodology. Study of the angle multiplex type of data storage system using what is known as "K-space" analysis (K being the "grating vector" of Solymar, et al.) shows that such factors dictate that the crystal volume cannot be utilized with optimum efficiency. A hologram, especially one with high data content, recorded at one angle will have crosstalk with a hologram at another angle unless the angles are adequately separated or the information is bandwidth limited.
Prior investigations have established a number of useful individual techniques without, however, resulting in material benefit in terms of the final product. For example, it is known that if writing and fixing simultaneously take place at high temperature, a larger electronic grating can be formed, which leads to an increase in fixed hologram diffraction efficiency. However, as will be shown later, the shortcoming of fixed gratings lies in the fixing process itself and not in the original writing process, as shown by the large effects obtained by Vladimirtsev, supra. Moreover, the holograms cannot be read out in an equally high temperature environment, and readout at ambient temperature requires compensation for thermal expansion effects which are considerable. Also, angle multiplexing by itself requires an extremely precise positioning system for both writing and reading, since the field of view of such holograms is extremely small. The present invention incorporates features applicable to this very high temperature method to obtain very large fixed photorefractive effects.
Using the simultaneous writing and fixing method just described, the RCA group had succeeded in storing up to 100 holograms with diffraction efficiencies of up to 30% in a 0.02% Fe-doped LiNbO.sub.3 crystal (D. L. Staebler, W. J. Burke, W. Phillips, and J. J. Amodei, "Multiple Storage and Erasure of Fixed Holograms in Fe-doped LiNbO.sub.3," Appl. Phys. Lett. vol. 26, pp. 182-184 (1975)). These results were considerably better than have been subsequently obtained by other groups attempting to duplicate the experiment. Through private communication with W. Phillips and reconstruction with him of the experimental setup, it is believed that they unknowingly used their heating system while the holograms were being written in a manner which is now understood to be advantageous in one respect in writing and fixing high resolution, high diffraction efficiency holograms.
The photorefractive effect produces only small changes of refractive index, of the order of 10.sup.-4 as shown by the RCA work, because of the limited extent of internal physical dislocation which it can induce. Therefore, if a recorded hologram is not efficiently readable with devices affording currently available signal-to-noise ratios, the potential of the photorefractive medium cannot be efficiently utilized or much more sensitive and expensive signal detectors must be used.
Therefore, there are a number of specific factors that must be considered in devising improved photorefractive devices and systems. These include the amount and nature of the incorporated dopant, the susceptibility tensor, the photovoltaic properties of the medium, the wavelength of the light and range within which the medium is photorefractive, the beam directions, the thickness of the medium, and diffusion and drift effects. Any and all of these factors can affect the resolution of the hologram and the ability to detect or respond to information or patterns contained within the medium.
Nonetheless, the theoretical capabilities of photorefractive media, combined with modern imaging technology, offer unique potential for data storage and processing. Media of comparable size to video, CD (audio), and mixed audio and video discs, but with orders of magnitude greater storage capacity, would then become feasible. Further, the greater bandwidths demanded by high definition television can be accommodated within a single photorefractive storage medium. For practical use, however, the system for reading this information out should not only be economical, but electronically controllable for high data rate, wide bandwidth responses.
The Scientific American article describes a variety of new and surprising applications of photorefractive media that are now being studied and developed. Photorefractive materials are in the general class of nonlinear optical devices, which are relatively recent subjects of investigation and implementation. Improved understanding of the phenomenon and improved processing methods offer possibilities for many new devices and systems for controlling or utilizing propagation of wave energy, whether it be in imaging, communication, or data processing systems. The great amount of research directed to the subject of photorefractive properties and holographic systems, in contrast to the relatively limited number of devices using photorefractivity, shows that a disparity exists between theory and practice. This was further affirmed by the observations of Solymar et al., supra as to the decline in optimism that new applications would be implemented. Solymar et al., supra, discuss some of the many possible photorefractive devices using volume holography that had been considered up to the time of publication, such as mass memories, couplers, lenses and multiplexers, but to the present there do not appear to be commercially suitable versions of such devices. When one considers that photorefractive materials function at the submicroscopic scale of electrons and ions, in contrast to the much larger microscopic resolution of optical contour variations, or magnetic or magneto-optic patterns on memory disks, it is evident that a great potential exists for high capacity memories. Moreover, as noted by Solymar et al., supra, there are now many other possibilities that arise from the ability to generate and modify monochromatic wave energy and optical images. Given the volumetric properties of photorefractive crystals, the different materials now available and the ever continuing demand for performance and lower cost in modern technology, a unified approach to the formation of high resolution, high density, essentially permanent holograms that are easily readable affords significant opportunity for expansion of the field.